A flight plan is the detailed description of the route to be followed by an aircraft in the context of a planned flight. It comprises in particular a chronological sequence of waypoints named and described by their position, their altitude and the time they are flown over. The waypoints constitute the reference path to be followed by the aircraft for the purpose of complying as well as possible with its flight plan. This reference path is a valuable aid both to the control personnel on the ground and to the pilot, for anticipating the movements of the aircraft and thus to ensure an optimum safety level, in particular in the context of maintaining criteria relating to the separation between aircraft. The flight plan is currently managed on board aircraft by a system named “Flight Management System” in English terminology, which will be referred to as FMS hereafter, which makes the reference path available to the flying personnel and to other on-board systems, such as display and acquisition interfaces. Essentially with safety in mind, it is therefore necessary to ensure that the aircraft follows the reference path described in the flight plan, at least in geographic terms and possibly in time terms. In order to do this, guidance procedures make it possible to slave the aircraft to the reference path. For example, the automatic pilot in “managed” mode generates maneuvers on the basis of the reference path made available by the FMS and executes them automatically. This makes it possible to follow the path corresponding to the reference path as closely as possible in four-dimensional space.
However, in certain situations, the pilot or the controller asks for an alteration or modification of the route. For example, the reference path can bring the aircraft to cross another aircraft, thus violating the lateral separation criteria. From his control center on the ground, the air traffic controller in charge of the flight notices the risk in advance because he knows the whole of the air situation within a large perimeter around the aircraft he is controlling. He therefore implements pre-established procedures for coordination between the ground and the aircraft, these procedures being currently grouped in English terminology by the term “Radar Vectoring”. In fact, the controller knows the position of an aircraft he is guiding by virtue of a radar and it is from this secured position that he derives the path to make the aircraft follow. The “Radar Vectoring” procedures can for example make it possible to ensure the crossing of two aircraft in conditions of optimum safety. They are based on a set of guidance commands or “instructions”, also predefined, which the controller passes on to the pilot. These instructions are wrongly grouped in English terminology by the term “clearance”. The pilot carries out manually the guidance instructions that he receives, one after the other, each time confirming their execution to the controller. Very often, the instructions are given exclusively orally by VHF radio, with the pilot also confirming the execution vocally. Certain recent systems use in parallel digital data links for exchanging some guidance instructions, these being for example in the textual format of the RTCA DO-258 standard, examples of which are given below.
In certain cases, the pilot manually updates the reference path of the flight plan in the FMS, in order that the latter is consistent with the guidance instruction. He does this in particular when the instruction received also indicates how to rejoin the reference path. For example, on receiving a direct flight instruction “PROCEED DIR TO [PT]” according to the RTCA DO-258 standard, where PT is a waypoint of the reference path, and which tells the pilot to head directly towards the point PT, the pilot deletes all of the waypoints before the point PT in the reference path that have not yet been passed. The direct flight instruction is generally given by a controller when the separation constraints are relaxed following a reduction in the density of air traffic. It allows the aircraft to take a shortcut without additional risk when traffic authorizes it. But it can also be envisaged that a direct flight instruction is given in the context of an avoidance procedure between two aircraft. In the short term, the reference path thus becomes a direct segment between the current position of the aircraft and the point PT, the waypoints beyond PT in the path remaining unchanged. The reference path, which is consistent with the real movements of the aircraft when the pilot updates the FMS by deleting the waypoints, can thus be used by the automatic pilot in “managed” mode and/or again be sent over the data link to the control centers on the ground.
Sometimes however, before even having reached the point PT according to the direct flight instruction, the controller can cancel his previous instruction “PROCEED DIRECT TO [PT]” or can issue an instruction to return to the flight plan “PROCEED BACK ON ROUTE” according to the RTCA DO-258 standard, which tells the pilot to rejoin the initial reference path of the flight plan. Unfortunately, the points which preceded the point PT and which have been deleted manually from the reference path following the direct flight instruction are no longer known by the FMS. Even the function of the FMS used to cancel an order, known in English terminology as the “undo” function, cannot be used for restoring them because it only applies to the temporary flight plan. Firstly, this function to cancel an order generally has a limited memory, storing only the latest orders given, or even only the very last one. Therefore it is not guaranteed that it can go sufficiently far back into the commands given, particularly if a large number of points have been deleted. In addition, even if the memory of the function to cancel an order is sufficient, another order has been able to be given to the FMS since the giving of the point deletion orders, this order probably having nothing to do with the application of the direct flight instruction. Most certainly, it must not be cancelled. However, the function to cancel an order of the FMS does not allow selective canceling; it allows only the cancellation of a more or less long consecutive sequence of the last orders given. Finally, even if no order had been given since the cancellation of the points, the restoration of the points by the function to cancel an order would not result in an acceptable situation. In fact, the flight plan would be retrieved exactly as it was before the giving of deletion orders. In particular, at least a portion of the dynamic information relating to the progression of the aircraft on the flight plan would suddenly correspond to the position of the aircraft before the deletion orders and no longer to its current position. For example, the first waypoints deleted would not be marked as passed after their retrieval, whereas they are probably far behind the aircraft. The algorithms for monitoring and updating the progression would risk failing to manage this unexpected inconsistency. Therefore the function to cancel an order of the FMS does definitively not respond to the operational problem presented here. An FMS of today cannot retrieve the points in question, which must at present be considered as definitively deleted. The initial reference path is therefore no longer available and the FMS cannot be of any help in the execution of a “PROCEED BACK ON ROUTE” instruction to return to the initial flight path in the current state of affairs.
A first solution is to recopy the active flight plan, which is the one currently being followed by the aircraft and made available to other systems by the FMS, into another flight plan called “secondary”, also managed by the FMS. Depending on the FMS system and on its version, from one to three secondary flight plans can be created and managed simultaneously, in addition to the active flight plan. They undergo exactly the same processing as the active flight plan; in particular the dynamic information relating to the progression of the aircraft is regularly updated according to the current position of the aircraft. However, only the active flight plan is made available to other systems such as the automatic pilot or the air traffic management systems on the ground via data links. The secondary flight plans are available only to the pilot and to the copilot by the intermediary of man-machine interfaces of the FMS, which make it possible to create, display, modify or delete secondary flight plans. Thus, on reception of the direct flight instruction mentioned above, the pilot firstly creates a secondary flight plan by recopying the active flight plan, and then only he deletes the waypoints of the active flight plan. On the subsequent reception of the instruction to return to the flight plan, the pilot activates the secondary flight plan which he has just created, which therefore becomes the active flight plan, the old active flight plan automatically becoming a secondary flight plan. The new active flight plan is up to date, in particular with regard to the dynamic information relating to the progression of the aircraft. The change from one flight plan to the other is virtually transparent and is done with relative continuity.
However, even if the FMS's of today provide all the services facilitating the creation and activation of a secondary flight plan from the active flight plan, this type of procedure is used in too many situations, not only that of the direct flight instruction followed by the return to the flight plan instruction. Thus, in view of the limited capabilities of FMS's to manage secondary flight plans, it can happen that recourse to this procedure is no longer possible for reasons of saturation of the FMS with secondary flight plans. The pilot is therefore presented with two alternatives. The first alternative is that the pilot can delete the waypoints of the active flight plan without the backup offered by a secondary flight plan. He then risks subsequently having to reenter them manually in the active flight plan, if he receives a return to flight plan instruction. If necessary, the pilot must use a paper version of the flight plan which allows him to find the previously deleted points. Sometimes he does not find all of them again, making this procedure rather uncertain. Then he must mark each of the reentered points that have been passed by the aircraft, this being done according to its current position. It is obvious that this manual procedure of reentering points constitutes a significant work overload which dangerously diverts the copilot from his nominal task. The second alternative is that the pilot can execute manually the direct flight instruction in visual flight without updating the flight plan in the FMS. The aircraft no longer follows the flight plan and no automatic procedure based on the flight plan can thereafter be engaged, in particular the automatic piloting in “managed” mode. Moreover, the systems on the ground receive a flight plan no longer corresponding to the real path of the aircraft. Here again, it is obvious that such a procedure is carried out to the detriment of safety. Without doubt it will no longer be operational when stricter standards which use a “4D” tube surrounding the route of the flight plan come into service.